This invention relates to a methodology for bonding together two microfabrication substrates.
Microelectromechanical systems are devices which are manufactured using lithographic fabrication processes originally developed for producing semiconductor electronic devices. Because the manufacturing processes are lithographic, MEMS devices may be made in very small sizes. MEMS techniques have been used to manufacture a wide variety of transducers and actuators, such as accelerometers and electrostatic cantilevers.
MEMS devices are often movable, they may be enclosed in a rigid structure, or device cavity formed between two substrates, so that their small, delicate structures are protected from shock, vibration, contamination or atmospheric conditions. Many such devices also require an evacuated environment for proper functioning, so that these device cavities may need to be hermetically sealed after evacuation. Thus, the device cavity may be formed between two substrates which are bonded using a hermetic adhesive.
Many devices require a transmissive material for the device substrate and/or the lid substrate, in order for the produced, emitted or altered electromagnetic radiation to be transmitted to or from the device cavity. Infrared emitters, detectors, attenuators, grating and mirrors for example, require encapsulation in infrared transmissive materials. Glass is a popular substrate material, however, glass is generally an insulating material. As a result, anodic bonding between two glass substrates is not possible, because the voltage cannot be applied to the insulating material properly. Also, it is often desired for the bonding material to be conductive, so as to provide an electrical pathway between structures on either substrate. Therefore a need exists for a packaging technology which can bond two optically transparent substrates with a conductive bond.
Anodic bonding of a glass substrate to a silicon substrate is known, wherein voltage and heat are applied between the glass substrate and the silicon substrate. The voltage applied promotes electrostatic attraction and the growth of an oxide layer between the silicon and the glass, which adheres the materials together. However, this method requires one of the substrates be a silicon or metal substrate, which, of course, is not transmissive to most portions of the electromagnetic spectrum, including the visible portion.
Accordingly, encapsulation of an optical device in a transparent device cavity which is hermetically sealed has been an unresolved problem.